1. Field of the Invention
The present invention relates to the art of digging a hole under the ground for installing an underground pipe such as a gas pipe, cable pie or water pipe. More particularly, the invention relates to a method using a propellant apparatus including a propellant head having a pressure-receiving face portion inclined relative to a head axis and a plurality of propellant cylinders pivotally and flexibly connected to the rear end of the propellant head and propelling the propellant apparatus under the ground by applying a thrust to the propellant apparatus from behind.
2. Description of the Related Art
The propellant apparatus is propelled in the earth as the pressure receiving face thereof receives the reaction force from the ground in association with the propellant movement, with the apparatus being guided to be flexed in the direction of the reaction force. Conventionally, in this type of underground propelling method, the propellant apparatus is constructed to be omnidirectionally flexible, so that in a curved propelling area the apparatus is propelled with its pressure receiving face portion being kept oriented to the opposite direction to the flexing direction. In a straight propelling area, on the other hand, the apparatus is propelled along a zig-zag path substantially along the planned straight path by reversibly switching over the orientation of the pressure receiving face portion.
With the above-described conventional method, as the propellant apparatus is omnidirectionally flexible, the apparatus can be propelled in any direction. On the other hand, if the orientation of the pressure receiving face portion is deviated from the planned direction and the apparatus is propelled under this condition, the apparatus will advance in this deviated direction. Accordingly, the steering control tends to be complicated.
The same problem as above may occur when the apparatus is propelled along a straight path, as the apparatus is apt to be flexed inadvertently. Each time the apparatus is deviated from the planned straight path, control becomes necessary to return the apparatus to the planned path. Hence, in this case too, the steering control tends to be complicated.
Then, a primary object of the present invention is to provide an underground propelling method capable of propelling the propellant apparatus along a planned path with high precision without necessitating complicated steering control scheme, thus solving the above-described drawbacks of the conventional method.